1. Field of the Invention
This invention relates to a magnetic disk apparatus equipped with a servo system for positioning the head on a target track of the disk by use of servo data prestored in the disk, and more particularly to a servo system having means for correcting the offset occurring at the time of positioning control.
2. Description of the Related Art
Conventional magnetic disk apparatus such as hard disk apparatuses have a servo system for moving the head to a target track (target cylinder) on the disk, and performing seek operation to position the same at the center of the target track. When the head is positioned on the target track, it performs data-read and -write operation in the data area of the track. In the case of using a plurality of surfaces of a single disk or plural disks, the word "cylinder" may be used in place of the word "track".
The servo system uses servo data prestored in the disk to perform positioning control for head seeking operation. The positioning control mainly consists of speed control performed at the time of moving the head to the target track, and track following control performed at the time of positioning the head at the center of the track. In a sector-servo-type servo system, data is stored in the servo area of each sector of a track, while in a dedicated-servo-type servo system, data is stored in a dedicated servo surface.
The head is supported by a head moving mechanism having an actuator and a voice coil motor (VCM), and is moved by the driving force of the VCM in a radial direction of the disk. The actuator supporting the head transmits a light signal to the head. Further, the actuator has a flexible printed cable (FPC) attached thereto for transmitting a read signal from the head to a read/write circuit. The FPC has an elastic force which will influence fine movement of the actuator.
The elastic force of the FPC will serves as an offsetting force when the servo system positions the head onto a target track, thereby deviating the head from the target track. This deviation makes longer the "settling time period" required to position the head onto the center of the target track. Accordingly, the seeking time period required is made longer, resulting in a reduction in access speed. Here, the "settling time period" is defined as the time period required to reliably move the head from a track located immediately before the target track, to the center thereof.
When positioning the head to the center of the target track by shifting the control mode from the speed control to the track following control, the servo system generates a position error signal PS based on the servo data to drive the VCM. More specifically, the servo system converts the signal PS to a driving current, to drive the VCM to position the head to the center of the target track.
FIGS. 6A-6C show initial response characteristics obtained at the time of the track following control. As is shown in FIG. 6A, a settling time period T1 is a time period required to position the head to the center of the target track, i.e., a time period required to cause the position error signal PS to have a wave level of 0. Supposing that the settling time period T1 is assumed for an offset value of 0, a settling time period T2 longer than the period T1 is assumed when a positive offset value occurs, as is shown in FIG. 6B, whereas a settling time period T3 longer than the period T1 is assumed when a negative offset value occurs, as is shown in FIG. 6C.
In order to correct the offset at the time of head positioning control to thereby shorten the settling time period, a method is developed in which offset data values are prestored in the ROM for correcting the offset, and used at the time of head positioning control. When converting the position error signal to a driving current and supplying the current to the VCM, the servo system adds or subtracts a current corresponding to the offset data value, to or from the driving current. Thus, as is shown in FIGS. 6B and 6C, offset in a positive or negative direction is corrected, thereby shortening the settling time period T2 or T3 to the reference settling time period T1.
Specifically, the ROM stores pre-offset data values as initial data values. Upon turning on the apparatus, the servo system positions the head to predetermined tracks and obtains an offset data value for each track on the basis of the driving current then supplied to the VCM. The offset data values thus obtained are stored in the RAM. If each offset data value stored in the RAM is identical to a corresponding pre-offset data value in the ROM, the pre-offset data value is stored as an optimal offset data value in the RAM. If, on the other hand, they are identical to each other, the offset data value obtained by actual positioning control and stored in the RAM is used as an optimal offset data value. This method is disclosed, for example, in U.S. application Ser. No. 766,778. In order to shorten the settling time period, the servo system generally adjusts the gain of the positioning control system (specifically, the gain of an operational amplifier for outputting the driving current), as well as setting optimal offset data values.
However, even if optimal offset data values are set upon turning on the apparatus, the actual offset value will be varied in accordance with variation in ambient temperature or in the input voltage of the operational amplifier for outputting the driving current. If the offset value exceeds an allowable range, offset cannot be corrected by use of the set offset data value at the time of the head positioning control.
In order to overcome the above, there are provided two conventional methods. In one of the methods, variation in temperature or voltage is monitored by a temperature sensor or a voltage sensor provided in the apparatus, and a new offset data value is obtained by performing the actual positioning control when the variation exceeds an allowable range, thus correcting the previous offset data value. In the other method, an optimal offset data value is set at regular intervals by performing the actual positioning control, without using such a temperature or voltage sensor. The first-mentioned method is disadvantageous in that a particular extra element such as a temperature or voltage sensor is necessary, while the other method is disadvantageous in that read/write access is interrupted at regular intervals, since no read/write access cannot be performed during the time period required to set a new offset data value.